This invention relates to a nuclear magnetic resonance (NMR) imaging device utilizing the periodical inversion of the field gradient and in particular to the time dependence of the field gradient.
In the NMR imaging generally a field gradient varying with time is utilized. In particular, by the echo-planar method (cf. Journal of Magnetic Resonance, 29, 355, 1978) or by the rapid chemical shift imaging method utilizing inverting magnetic field (U.S. patent application Ser. No. 750,475, field July 1, 1985, now U.S. Pat. No. 4,689,568, issued Aug. 25, 1987), signals are observed by applying a field gradient to a specimen while inverting it periodically. By these methods it is necessary to utilize a precise rectangular wave for the magnetic field. However, it is difficult to control precisely the waveform of the field gradient produced in practice, including characteristics of a field gradient generating system. Such imprecision in the control exerts undesirable influences on the image quality. In order to compensate this effect, a method, by which sampling is effected with unequal intervals (Journal of Physics, E17, 612, 1984), a method, by which data are sampled with an equal interval and then Fourier-transformed after having multiplied them by appropriate weight coefficients (cf. Journal of Magnetic Resonance, 42, 193, 1981), a method, by which an image is reconstructed by forming an appropriate linear sum of sampled data after having Fourier-transformed them (cf. Applied Physics, 22, 257, 1980), etc. have been proposed. By either of these methods it is necessary to know previously the waveform of the field gradient generated in practice.
As a method for obtaining this waveform, a method utilizing the ratio of the derivative dF(t)/dt of the NMR signal F(t) to F(t) has been proposed. However this method had problems that requirements for the arrangement and the form of specimens were very severe, that it was sensitive to noises, because the derivative was used, etc.